It is generally known that process plants, such as refineries, chemical plants or pulp and paper plants, consist of numerous process control loops connected together to produce various consumer products. Each of these process control loops is designed to keep some important process variable such as pressure, flow, level, or temperature, within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates load disturbances that affect the process variable and control of the process control loops within the plant. To reduce the effect of these load disturbances, process variables are detected by sensors or transmitters and communicated to a process controller. A process controller processes this information and provides changes or modifications to the process loop to get the process variable back to where it should be after the load disturbance occurs. The modifications typically occur by changing flow through some type of final control element such as a control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired control or set point.
It is generally understood that various control valve configurations may be specifically applicable for certain applications. For example, when a quick-opening valve with a narrow control range is suitable, a rotary control valve, such as a butterfly valve, may be used. Alternatively, when precise control over a large control range is required, a sliding stem control valve may be used. Thus, when designing a process, the process engineer must consider many design requirements and design constraints. The style of valve used and the sizing of the valve can have a large impact on the performance of the control valve in the process control system. Generally, a valve must be able to provide the required flow capacity when the valve is at a specific open position. Flow capacity of the valve is related to the style of valve through the inherent characteristic of the valve.
The inherent characteristic is the relationship between the valve flow capacity and the valve travel when the differential pressure drop across the valve is held constant. Under the specific conditions of constant pressure drop, the valve flow becomes only a function of the valve travel and the inherent design of the valve trim. These characteristics are called the inherent flow characteristic of the valve. Inherent valve characteristic is an inherent function of the valve flow passage geometry and does not change as long as the pressure drop is held constant. Most sliding stem valves have a selection of valve cages or plugs that can be interchanged to modify the inherent flow characteristic. Knowledge of the inherent valve characteristic is useful, but the more important characteristic for purposes of process optimization is the installed flow characteristic of the entire process, including the valve and all other equipment in the loop. The installed flow characteristic is defined as the relationship between the flow through the valve and the valve assembly input when the valve is installed in a specific system, and the pressure drop across the valve is allowed to change naturally, rather than being held constant.
Because of the way it is measured, as defined above, the installed flow characteristic and installed gain are really the installed gain and flow characteristic for the entire process. Typically, the gain of the unit being controlled changes with flow. For example, the gain of a pressure vessel tends to decrease with throughput. Therefore, because the valve is part of the loop process as defined here, it is important to select a valve style and size that will produce an installed flow characteristic that is sufficiently linear to stay within the specified gain limits over the operating range of the system. If too much gain variation occurs in the control valve itself, it leaves less flexibility in adjusting the controller. For example, if the low end of the gain range is too low, a lack of responsiveness can create too much variability in the process during normal operation. However, there is also a danger in letting the gain get too large. The loop can become oscillatory or even unstable if the loop gain gets too high, thus, valve sizing becomes important. For example, it is common to oversize a valves when trying to optimize process performance through a reduction of process variability. Oversizing the valve hurts process variability in two ways. First, the oversized valve puts too much gain in the valve, leaving less flexibility in adjusting the controller. Best performance results when most loop gain comes from the controller. If the valve is oversized, making it more likely to operate in or near this region, this high gain can likely mean that the controller gain will need to be reduced to avoid instability problems with the loop. This, of course, will mean a penalty of increased process variability.
Because an oversized valve produces a disproportionately large flow change for a given increment of valve travel, this phenomenon can greatly exaggerate the process variability associated with dead band due to friction. Regardless of its actual inherent valve characteristic, a severely oversized valve tends to act more like a quick-opening valve, which results in high installed process gain in the lower lift regions. In addition, when the valve is oversized, the valve tends to reach system capacity at relatively low travel, making the flow curve flatten out at higher valve travels. When selecting a valve, it is important to consider the inherent characteristic, and valve size that will provide the broadest possible control range for the application. Adequate flow capacity can be achieve by simply selecting a larger control valve, but oversizing the valve can cause problems. Thus, minimizing the body size of a valve for a particular application provides many benefits.
Minimizing the valve size reduces the cost of the valve itself and reduces the cost of the actuator that controls the valve. Additionally, some process control applications require a valve to maximize flow in two directions, often called a “bi-directional flow application.” In a typical bi-directional valve, there is a vertical segment where fluid flows upward. Therefore, flow in one direction is often referred to as flow-up and flow in the other direction is referred to as flow-down. In a majority of applications, valve selection is predicated on the preferred direction of flow through the valve. Port guided globe valves are popular for bi-directional flow applications because they can be utilized regardless of flow direction. A port guided plug is supported by the skirt as the plug slides up and down, guided along an annular valve seat in the valve body irrespective of flow direction. In some cases the annular valve seat serves a dual role as a bearing surface for the plug skirt and as a sealing surface for mating with the sealing surface on the plug. In particular, the skirt acts as a guide to stabilize the valve plug within the valve as fluidic forces place a side load on the plug. A smaller valve stem provides multiple benefits including minimization of the force required to move the plug because there is less friction on the stem from the valve stem packing and seal. Smaller valve stems are also easier to seal because there is less force on the seal due to reduced surface area. Minimizing the size of the valve stem also minimizes the size of the actuator required to move the valve plug due to reduced operating friction. Reduced friction also provides improved plug reaction time and better overall valve performance. One inherent problem with utilizing a port guided globe valve is that the valve plug is typically not fully removed from the valve seat. As a result, skirt material obstructs the flow path and reduces the amount of flow in a full open condition. In addition to decreasing maximum capacity by decreasing the diameter of the flow path, the skirt obstruction results in hydrodynamic drag. Thus, the obstruction presented by the skirt prevents the valve from producing the maximum flow properties found in other valve types having the same port size.